Osteoarthritis had forced her to have her right hip replaced in 1998 and doctors warned that her left hip would most likely meet the same fate. Eventually, the ache progressed to a constant throb that even a double dose of her pain medication didn't stop. She couldn't walk around the block, and even had to ask her husband to shave her legs.

''I just couldn't bend over anymore,'' said the 47-year Dallas homemaker, who had the operation last month. ''It was time.''

Advancements in surgery and prostheses combined with a dearth of medicines for osteoarthritis are nudging doctors to perform more hip and knee replacements on baby boomers. Although the procedures are still mostly performed on people over 65, a growing number of boomers, who range from 38 to 56, are getting implants.

According to the American Academy of Orthopaedic Surgeons, baby boomers had 35,000 hip replacements or 21 percent of the procedures in 2001, the last year for which figures are available. That's up from 16 percent of all procedures in 1997.

Likewise, boomers had 48,000 knee replacements or 15 percent of the total 2001, up from 12 percent in 1997.

Doctors estimate that over 90 percent of joint replacements are done because of osteoarthritis, which affects nearly 21 million people and is the most common form of arthritis. Osteoarthritis is a degenerative disease characterized by the breakdown of a joint's cartilage, and is caused by a variety of factors including injuries, obesity and genetics. The breakdown causes bones to rub against each other, resulting in pain and loss of movement.

The incidence of arthritis increases as people age. In 1997, the Centers for Disease Control and Prevention estimated 43 million Americans suffered with arthritis, up from 37.9 million in 1990. A 2001 report estimated 70 million Americans suffered with the disease although the study was conducted differently than earlier versions.

Doctors say baby boomers seem to have arthritis symptoms at a younger age than previous generations, probably because of their active lifestyles. With a motto of ''no pain, no gain'' boomers have embraced jogging, aerobics, skiing and other activities.

''This is a generation that played hard,'' said Dr. Nicholas A. DiNubile, an orthopedic surgeon in Havertown, Pa. ''Their parents didn't play sports. They didn't get early injuries.''

A decade or so ago, doctors and patients chose to put off implants as long as possible because prostheses would only last about 10 years and replacement surgery becomes less effective and more dangerous each time it is done. Now prostheses are expected to last 25 years, so doctors are more willing to give them to younger patients.

''We know the time will come when they may have to have a future operation but now it is 20 or 30 years down the line,'' said Dr. Robert Bucholz, chairman of the orthopedic surgery department at the University of Texas Southwestern Medical School in Dallas. ''Now the boomers want to be able to keep up with their kids and grandkids.''

While the implants have improved, arthritis medications have not. Over the last decade, new pain relievers that are gentler on the stomach such as Vioxx and Celebrex have been introduced, but there are no medicines to stop the progression of osteoarthritis. There are medicines for rheumatoid arthritis but it is a much less common form of the disease, affecting about 2 million Americans.

''Boomers are less willing to live with symptoms. They are more aggressive about their medical treatments,'' DiNubile said.

The problem with giving boomers new hips and knees, doctors say, is convincing them to adjust active lifestyles that may have contributed to the need for surgery.

''You have to tell the patients all the time that these are not the joints that God gave you,'' said Dr. Frank Kelly, an orthopedic surgeon in Macon, Ga. ''I tell them in person, I show them videos, I write it down. I hope they listen.''

Bill Mc Mullen has been a good patient. Before his right knee was replaced in 2001, he had seven procedures to remove bones chips to alleviate the pain and postpone surgery as long as possible. He also retired from his physically demanding job as a steamfitter to avoid placing unnecessary burdens on the implant.

''I enjoyed the job,'' said the 56-year old suburban Philadelphia resident. ''But I don't want to abuse my knee.''

Mc Mullen said the surgery meant a change from crippling pain to finally being able to sleep through the night. ''That is a big deal,'' he said.

A study released last year showed that patients who opted to postpone joint replacement surgery the longest had more pain and less mobility than patients who didn't wait. The 222 patients in the study were in their 60s and 70s, but there are implications for boomers too, said its author, Dr. Jeffrey Katz, an assistant professor of rheumatology at Brigham and Women's Hospital in Boston.

''I think the medical profession held patients back from this surgery,'' Katz said. And while he doesn't advocate arthritis patients undergoing surgery right away, he said the study should encourage doctors and patients not to wait until pain is unbearable.

''There is no need to accept joint pain as a fact of life,'' Katz said.